Vaporization process



NOV. 29, 1966 J, own ET AL 3,288,702

VAPORIZATION PROCESS Filed March 29, 1965 EDWARD J 00170 62/ 61/725 Inventors e fl wa w Poient Attorney process.

United States Patent 3,288,702 VAPORIZATION PROCESS Edward J. Dowd, Summit, and Eli Cutler, Scotch Plains,

NJ., assignors to Esso Research and Engineering Company, a corporation of Delaware Filed Mar. 29, 1965, Ser. No. 443,454 5 Claims. (Cl. 208-48) This invention relates to an improved process for the heating of hydrocarbons. With more particularity it relates to the vaporization of a hydrocarbon feedstream. Still more particularly it pertains to a process wherein a feedstream may be completely vaporized without going through the dry point in a furnace. This is accomplished by recycling liquid, previously obtained from the same feedstream back to the furnace.

The use of furnaces to heat hydrocarbon fuels for various processes is well known. For example, it is well known to preheat hydrocarbon feed in a furnace prior to cracking, hydrotreating, hydroforming, and molecular sieve separation to name just a few of a great variety of situations. In all of these areas one common problem has been faced. As the oil becomes heated to progressively higher temperatures, the hydrocarbons will vaporize. At a given point all the oil has vaporized and no liquid remains from the original hydrocarbon stream. At this point a final vaporization or dry point has occurred. That is to say, no liquid remains, the hydrocarbon is entirely in the vapor state. Concurrently, with the complete vaporization of the hydrocarbon, a great deal of coking or deposition of fouling or plugging materials occurs within the furnace tubes. The tubes of the furnace will eventually become so badly clogged or metal temperatures will become so high that the furnace must be shut down completely in order that the tubes be cleaned so that they are again suitable for operation. To be sure, coking and fouling can occur prior to the achievement of the dry point; however, they tend to become a great deal more severe at this time.

Numerous attempts have been made to circumvent this problem. However, they have met only with the most limited success.

Among the earliest attempt at solution of this problem was the maintaining of high pressure levels in the furnace tubes to suppress vaporization. This did not represent a satisfactory solution since it was quite difficult to control and degradation tended to take place in the liquid phase.

' Another attempt to solve the problems caused by the dry point involved the partial vaporizing of the feedstream along with the adding of an extraneous gas stream to promote vaporization. Any limited success achieved by this process was counteracted by difliculty in controlling the Furthermore, the additional heat required along with fractionation which is needed to separate the vapor stream, entailed a great deal of expense.

An extraneous high boiling liquid may also be added to the feed; this was tried several times.

One of these attempts, is worthy of some note. In US. Patent 2,472,669, a method for preventing or minimizing coke formation in preheater tubes is described. This patent pertains to a process wherein complete vaporization of a charge oil is desired while coking is to be minimized. This is accomplished by adding, along wth the feed stock to be treated, 1 to by volume of a substantially non-coking high boiling oil. The oil is of such boiling range that it remains in the liquid state during passage through the tube heater. Crude oil fractions utilized would have an average boiling point about 1050 F. This invention may be somewhat successful in that the presence of this heavier liquid will serve to wash away coke deposits which would normally form on the tubes However, this process is not without serious drawbacks. There is some question as to the added liquids own coking tendencies. When utilized in a thermal or catalytic cracking process the liquid would have to be discarded; it could not be recycled to the heater after each pass. This is because it would contain carbon residue from the added heavy oil as well as the portions of the original charge oil which have greatest coke-forming tendencies.

Furthermore, some of this added heavier liquid will be present in the removed vapor and this will cause problems of product purity as well as possible catalyst poisoning or possible increase-d corrosion. The product purity of the vaporized portion will naturally be lowered by the presence of this extraneous material. This material could also introduce traces of elements or compounds not found in significant amounts in the charge stock, which would lead to poisoning of catalyst or excessive corrosion of the materials of construction.

7 Expensive separation techniques would be needed to rid the vapor stream completely of the vaporized heavy liquid or to rid the heavy liquid of poisons or corrosive materials before its addition.

Also, some of the higher boiling constituents of the charge stock would be found in the separated liquid and thus be lost from the charge stock.

According to this invention the problems encountered in the prior art have now been resolved. This invention allows the complete vaporization of a feedstream without presenting traditional problems of furnace tube coking or fouling which reaching the dry point has presented in the past. This is done by sending-the partially vaporized material from a furnace to a drum where after equilibrium is obtained between vapor and liquid phases all of the liquid is recycled back to the feed. When the system is in practical equilibrium, there is no increase or decrease of liquid in the drum the vaporized material going overhead at the drum will have the same composition and rate as the feed. This invention eliminates the potential coke formation in the Ifurnace when the feed goes through the dry point in conventional practices, since the recycled liquid provides a liquid wash constantly for the tubes. This material has the lowest coking or fouling tendency of any material which could be used as liquid wash while accomplishing complete vaporization of a feedstream. With the equilibrium having been obtained, the vaporized feed maintains substantially the same constituents. This invention, therefore, provides the advantage of the presence of a liquid washing the furnace tubes and at the same time does not require the addition, separation or purification of a heavier, extraneous liquid. Because of this the vaporized feed being removed may be kept at a relatively constant composition, the eflluent temperature is the lowwithin the furnace.

est attainable when using a liquid wash technique, and

there is no need to separate a contaminant from it.

It should be emphasized that after equilibrium is obtained, the feed entering the furnace has the same composition as the vapor leaving the drum. Equilibrium is obtained when the liquid level in the drum remains substantially constant.

The attached figure is a schematic view of an embodiment of the instant invention.

Turning to the figure, a feedstream is passed through line 1 into furnace 2. The feedstream passes into the tubes of the furnace which are designated by the nu meral 3. The feedstream may be any hydrocarbon frac tion boiling up to 700 F. Falling within this range would by gasolines, kerosenes, and a variety of low and medium boiling gas oils. As an example, the feed may be considered to be a kerosene fraction boiling between 375 and 575 F. This fraction is passed into furnace 2 which may be a cabin, box, vertical cylindrical, batch, Bessemer, booth-hall, ceramic, electric or any other well known-type furnace. Any amount from 10 to about 95% of the feed is vaporized within the furnace depending on the temperature at which the furnace is maintained. The vapor-liquid mixture is removed from the furnace through line 4 and passed into settler vessel 5. Vaporized feed is removed overhead from the vessel 5 through line 6. Liquid accumulateswithin the vessel 5. This liquid fraction consists of the same constituents of the kerosene feed but in a distribution which is richer than the feed in constituents boiling near 575 F. and leaner than the feed in constituents boiling near 375 F. Liquid accounts for between 5 and 80% of the entire feedstock which was passed into the furnace. This liquid is removed from the bottom region of vessel 5 through line 9, from whence it is passed through pump 7, then through line 8 which joins with feedstream line 1. Surplus liquid may be removed through line 10. Vaporized feed is removed overhead through line 6. This process is continued until an equilibrium is obtained. An equilibrium is obtained when this vaporized feed has substantially the same content as the feed stock. At equilibrium the liquid level in vessel 5 remains substantially constant.

When this equilibrium is obtained, the liquid may be recycled from the bottom of vessel 5 through line 9, pump 7 and line 8 into line 1. About 20% of the feed stock including recycled material which passes through the furnace is recovered as liquid and recycled back to mix with the fresh feedstream. As mentioned above, because of this process a vapor phase may be removed which is substantially identical in constituents from the original feed stock which is passed through the furnace. At the same time, no significant coking deposite occurs within the furnace.

Example 1 In a specific embodiment of this invention a kerosene feed stock boiling approximately between 320 F. and 520 F. was utilized. This feed stock was introduced into an apparatus substantially identical to the figure described above. The feed was introduced at a rate of 18,000 barrels per day and the furnace was started at a temperature of about 475 F. About 80% of the feed was vaporized and the remainder stayed in the liquid phase. The feed stock was passed through the furnace and temperatures were carefully raised to maintain an essentially constant quantity of liquid in vessel 5, until equilibrium was established at 615 F., the dew point temperature of the feed at the operating pressure of about 100 pounds per square inch. The liquid fraction accumulated in vessel 5 consisted of the same constituents as the kerosene feed. This fraction was recycled and it comprised about 25% of the original feedstock. After about 13 weeks of operation no visible effects of coking were apparent.

Example 2 In this example the same conditions as in Example 1 are utilized except that no portion of the liquid is recycled. After about 13 weeks, visible signs of coking are apparent.

This indicates that the recycling of the liquid serves to substantially eliminate all problems with coking.

Example 3 In this example the exact conditions of Example 1 are utilized except that there is no recycle of the liquid. The entire feed is vaporized. A heavy oil boiling at approximately 1050 F. is continuously added to the furnace feed and separated in drum 5. This process is unsatisfactory for the following reasons:

Trace heavy boiling material leaves with the vapor and cokes on the adsorbent affecting run length, capacity, selectivity and adsorbent life. 7

Treat gas requirements to adsorbent increase, which requires expensive compressor horsepower.

' Some of the valuable feed (of jet fuel quality) is'downgraded and disposed of with the heavy oil.

' Coil outlet temperature would be raised. This would favor decomposition of traces of treat gas (dissolved in feed) which would attack furnace tube materials excessively and would dilute treat gas, a significant process debit.

All available oils in the 1050" F.+ boiling range would require complete desulfurization and removal of other potential adsorbent poisons such as vanadium, salt, etc.

Although the invention has been described with some particularity, it is intended. only to be limited by the attached claims.

What is claimed is:

1. An improved hydrocarbon heating process which comprises passing a liquid hydrocarbon feedstream into a furnace zone, vaporizing at least a portion of said feedstream, passing said partially vaporized feedstream into a vessel, continuing to pass said feedstream into said vessel until an equilibrium is obtained in the vessel between liquid and vapor, raising the temperature within said zone to a temperature which is at least as high as the maximum boiling point of the said hydrocarbon feedstream but not sufliciently high to vaporize more than a portion of the recycled liquid removing vapor from the vessel, vapor having substantially the same composition as said feedstream, recycling said liquid into admixture with said feedstream whereby the presence of said liquid prevents the formation of coke deposits within said furnace.

2. The process of claim 1 wherein the said feed stock is a hydrocarbon boiling up to 700 F.

3. An improved hydrocarbon heating process which comprises passing a hydrocarbon feed stock boiling at a maximum of about 700 F. into a furnace zone, whereby a maximum of of the fed stock is vaporized and the remainder remains in the liquid phase, passing said partially vaporized feed stock into a vessel, raising the temperature in the furnace zone to at least about 700 F. continuing to pass said partially vaporized feed stock into said vessel until the liquid level within saidvessel remains substantially constant, removing said liquid from said vessel and recycling it into admixture with said feed stock whereby the formation of coke within said furnace is prevented, withdrawing vapor from said vessel having substantially the same composition as said hydrocarbon feed stock. 1

4. The process of claim 3 wherein said feed stock is a naphtha fraction.

5. The process of claim 3 wherein said feed stock is a kerosene fraction.

References Cited by the Examiner UNITED STATES PATENTS 6/ 1949 Mathy 208-48 7/1957 Barr et a1. 196108 

1. AN IMPROVED HYDROCARBON HEATING PROCESS WHICH COMPRISES PASSING A LIQUID HYDROCARBON FEEDSTREAM INTO A FURNACE ZONE, VAPORIZING AT LEAST A PORTION OF SAID FEEDSTREAM, PASSING SAID PARTIALLY VAPORIZED FEEDSTREAM INTO A VESSEL, CONTINUING TO PASS SAID FEEDSTREAM INTO SAID VESSEL UNTIL AN EQUILIBRIUM IS OBTAINED IN THE VESSEL BETWEEN LIQUID AND VAPOR, RAISING THE TEMPERATURE WITHIN SAID ZONE TO A TEMPERATURE WHICH IS AT LEAST AS HIGH AS THE MAXIMUM BOILING POINT OF THE SAID HYDROCARBON FEEDSTREAM BUT NOT SUFFICIENTLY HIGH TO VAPORIZE MORE THAN A PORTION OF THE RECYCLED LIQUID REMOVING VAPOR FROM THE VESSEL, VAPOR HAVING SUBSTANTIALLY THE SAME COMPOSITION AS SAID FEEDSTREAM, RECYCLING SAID LIQUID INTO ADMIXTURE WITH SAID FEEDSTREAM WHEREBY THE PRESENCE OF SAID LIQUID PREVENTS THE FORMATION OF COKE DEPOSITS WITHIN SAID FURNACE. 